Encapsulated thermostatic switch having a heater disposed in the stem



W. C. RICKER ETAL ENCAPSULATED THERMOSTATIG SWITCH HAVING A HEATER DISPOSED IN THE STEM Filed NOV. 12, 1964 Sept. 13, 1966 WILLIAM C. RICKER GEORGE H. SIMPSON INVENTORS ATTORNEY United States Patent 3,272,944 ENCAPSULATED THERMOSTATIC SWITCH HAV- ING A HEATER DISPOSED IN THE STEM William C. Ricker, Waldoboro, and George H. Simpson,

Boothbay, Maine, assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Nov. 12, 1964, Ser. No. 410,467 2 Claims. (Cl. 200-122) This invention relate-s to thermostatic switches and more particularly to miniaturized, glass-enclosed, hermetically-sealed, bimetal-activated devices designed particularly for use as electrical malfunction detectors and circuit protectors. These circuit breakers are designed to handle specified hold currents continuously and to trip the circuit at certain minimum current values.

In one form of construction, a tungsten filament is mounted across the lead-in wires of the device. Under normal conditions, the bimetal armature is closed, thus by-passing the filament. With an overload, the bimetal heats to a point where the contacts open, thus impressing essentially full voltage across the filament. This causes the filament to become incandescent and produce sufficient heat to hold the bimetal and contacts in an open position until the voltage is removed. This arrangement is not entirely satisfactory for several reasons. Filament mounting is a relatively expensive operation and the relative location of the filament is critical, particularly on high current types which may recycle on occasion. The filaments must run very hot since most of the heat is transferred by radiation. With proper breaker operating characteristics, filament life tends to be short. Circuit breakers with filament heaters are not particularly suited for use in applications where vibration is a factor because of the probability of filament turns being shorted out.

In view of the foregoing, the principal object of this invention is to provide a new and improve-d resistive heating element to replace the tungsten filament.

In accordance with the principles of our invention, a body of powdered high resistance material is located on the top of a post or base member of insulating material through which a pair of spaced lead-in wires extend. This body of high resistance material spans the lead-in wires. When the circuit breaker contacts are open, the external voltage forces a small current through this resistance and generates heat. This heat is conducted by the lead-in wires to the bimetal element keeping it in the open position indefinitely.

In the accompanying drawing, the single figure is an elevational view in section of a specific embodiment of a circuit breaker illustrating our invention. The illustrated circuit breaker comprises a switch mount 1 hermetically sealed inside a tubular glass envelope 3. One end of the envelope 3 is characterized by an exhaust tip 5 and the other end has an external press 7 through which lead-in wires 9 and 11 of mount I extend. The mount 1 comprises. a glass stem 13 through which the lead-in wires 9 and 11 extend. One end of a bimetal armature 15 is secured to lead-in wire 11. A compensator 17 is secured to the other end of the armature 15. A contact 19 on the compensator 17 is normally closed with respect to lead-in wire 9.

The top of the stem 13, between the points of emergence of the lead-in wires 9 and 11, is provided with a cavity 21 which is filled with a quantity of powdered high resistance material 23, such as graphite for example. The graphite 23 is applied as a suspension in a suitable binder which has a high temperature break-down point and adheres well to the resistance material and the stem 13. The body of graphite 23 bridges the lead-in wires 9 and 11. Thus, when the contact 19 is opened with respect to the lead-in wire 9, the external voltage forces a small current through the graphite 23 and generates heat. This heat is conducted to the bimetal armature 15, keeping it in open position indefinitely. Since the body of resistance material is applied in a suspension, it can be deposited on the stem with an automatic application system. The material is held in place by the binder and thus is relatively insensitive to vibration. It runs at a low temperature since the major portion of the heat generated is conducted to the bimetal armature. The presence of air or other foreign gas in the envelope 3 is of no consequence in the case of the graphite material, whereas it is quite harmful to a tungsten filament because of the vaporization of the tungsten which it causes.

By way of example, we apply a body of powdered high resistance material as a suspension in a suitable binder. More particularly, we use a 2.00 grams mix comprising about 43 grams of graphite and about 157 grams of silica or powdered quartz. This mix is disposed as a suspension in about grams of a sodium silicate. The mix has two components, a high resistance material such as graphite, and a filler of refractory material such as powdered quartz. This filler should have about the same expansion coefficient as graphite and possess good insulating properties. Resistance properties of the mix can be varied by changing the filler-graphite ratio. The suspension or carrier is a highly adhesive sodium silicate and is quite fluid.

Drying time of the body of powdered high resistance material 23 in place in the cavity 21 provided therefor can be regulated by varying the amount of water in the sodium silicate carrier. Good results may 'be obtained with about 48 grams of sodium silicate in about 72 grams of water.

What We claim is:

1. A thermostatic switch comprising: a glass envelope; a .stem of insulating material disposed in said envelope, said stem having a cavity formed in the top thereof; a pair of spaced lead-in wires extending through said stem and through an end of said envelope; a bimetal armature secured to one of said lead-in wires; a compensator secured to said armature; an electrical contact secured to said compensator, said contact being normally closed with respect to the other of said lead-in wires; and a body of powdered high resistance material disposed in said cavity formed in said stem and bridging said lead-in wires.

2. A thermostatic switch comprising: a glass envelope; a stem of insulating material disposed in said envelope; a pair of spaced lead-in wires extending through said stem and through an end of said envelope; a bimetal armature secured to one of said lead-in wires and normally closed with respect to the other; and a powdered mix disposed on said stem and bridging said lead-in wires, said mix comprising a quantity of high resistance material and a filler of refractory material.

References Cited by the Examiner UNITED STATES PATENTS 2,336,504 12/ 1943 Ruben 200-122 2,403,803 7/ 1946- Kearsley. 2,502,203 3/1950 Carpenter et a1. ZOO-113.5

BERNARD A. GILHEANY, Primary Examiner. T. D. MACBLAIN, Assistant Examiner. 

1. A THERMOSTATIC SWITCH COMPRISING: A GLASS ENVELOPE; A STEM OF INSULATING MATERIAL DISPOSED IN SAID ENVELOPE, SAID STEM HAVING A CAVITY FORMED IN THE TOP THEREOF; A PAIR OF SPACED LEAD-IN WIRES EXTENDING THROUGH SAID STEM AND THROUGH AN END OF SAID ENVELOPE; A BIMETAL ARMATURE SECURED TO ONE OF SAID LEAD-IN- WIRES; A COMPENSATOR SECURED TO SAID ARMATURE; AN ELECTRICAL CONTACT SECURED TO SAID COMPENSATOR, SAID CONTACT BEING NORMALLY CLOSED WITH RESPECT TO THE OTHER OF SAID LEAD-IN WIRES; AND A BODY OF 